1. Field of the Invention
This invention relates to a phase change optical recording medium, and a method for overwriting such medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Ge--Sb--Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of high power (recording power) that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
In the optical recording medium of phase change type, dielectric layers are generally formed on opposite sides of the recording layer. Requirements for the dielectric layers are:
(1) the dielectric layers should be capable of protecting the recording layer and the substrate from heat histerisis as a result of the laser beam irradiation; PA1 (2) the dielectric layers should be capable of amplifying the reproduced signal by making use of optical interference effect of the lights reflected from boundaries between the layers; and PA1 (3) the recording and erasing properties can be regulated by adjusting thermal conductivity and the like of each dielectric layer. PA1 the first dielectric layer comprises a dielectric layer 1a on the side of the substrate and a dielectric layer 1b on the side of the recording layer, and the dielectric layer 1a contains zinc sulfide and silicon oxide as its main components and the dielectric layer 1b contains silicon nitride and/or germanium nitride as its main component(s); PA1 the second dielectric layer contains zinc sulfide and silicon oxide; silicon oxide; or oxide of a rare earth metal as its main component(s); and PA1 the dielectric layer 1b has a thickness of 0.5 to 40 nm, and ratio (t.sub.1b /t.sub.1) of thickness (t.sub.1b) of the dielectric layer 1b to the thickness (t.sub.1) of the first dielectric layer is up to 0.5. PA1 the first dielectric layer comprises a dielectric layer 1a on the side of the substrate and a dielectric layer 1b on the side of the recording layer, and the dielectric layer 1a contains zinc sulfide and silicon oxide as its main components and the dielectric layer 1b contains silicon nitride and/or germanium nitride as its main component(s); PA1 the second dielectric layer comprises a dielectric layer 2a on the side of the recording layer and a dielectric layer 2b on the side of the reflective layer, and the dielectric layer 2a contains zinc sulfide and silicon oxide as its main components and the dielectric layer 2b contains silicon oxide or oxide of a rare earth metal as its main component; and PA1 the dielectric layer 1b has a thickness of 0.5 to 40 nm, and ratio (t.sub.1b /t.sub.1) of thickness (t.sub.1b) of the dielectric layer 1b to the thickness (t.sub.1) of the first dielectric layer is up to 0.5. PA1 the medium is overwritten with a laser beam modulated in three power levels of P.sub.P (peak power); P.sub.B1 (bias power 1) lower than the P.sub.P ; and P.sub.B2 (bias power 2) lower than the P.sub.B1 ; and PA1 the laser beam for the record mark formation is pulse modulated such that the peak power is P.sub.P and the bottom power is P.sub.B2, and the power level is reduced to P.sub.B2 after the irradiation of the last pulse and increased to the erasing power level of P.sub.B1.
Typical dielectric layers which meet such requirements are those containing highly refractive ZnS as their main component. For example, Japanese Patent Application Kokai (JP-A) No. 103453/1988 discloses an optical information recording member having a dielectric layer containing a mixture of ZnS and SiO.sub.2. The merits described therein include increase in sensitivity for the power of incident light upon recording, and increase in the number of erasing/overwriting operations of the dielectric material. The increase in the sensitivity is said to have been realized by optimizing thermal constant of the dielectric layer, and the increase in the number of erasing/overwriting operations is said to have been realized by preventing the alteration in the nature of the dielectric layer. JP-A 103453/1988 discloses that SiO.sub.2 /(ZnS+SiO.sub.2) is preferably in the range of 10 to 30 mol % since the laser energy required for the recording and the erasure is minimum when SiO.sub.2 /(ZnS+SiO.sub.2) is in such range.
However, in the optical recording medium of phase change type wherein the recording layer comprising Ge--Sb--Te based material or the like is sandwiched by dielectric layers containing ZnS as their main component, C/N reduces with the repeated overwriting operations, and the medium becomes unoverwritable after approximately several thousand times. A major cause for the decrease in C/N with the repeated overwriting operations is believed to be influence of the alteration in the composition of the recording layer due to element diffusion between the adjacent dielectric layer.
JP-A 64937/1990 describes improvement of adaptability to repeated recording operations by providing a heat-resistant protective layer on one surface or both surfaces of the recording medium, and a protective layer having a bulk modulus smaller than that of the heat-resistant protective layer on at least one surface of the heat resistant protective layer. The exemplary materials disclosed therein for constituting the protective layer of smaller bulk modulus are MoS.sub.2, ZnS, ZnSe and the like, and the exemplary materials disclosed therein for constituting the heat-resistant protective layer are silicon carbide, silicon nitride, and aluminum oxide. In Example 2 of JP-A 64937/1990, SiN.sub.x layers of 20 nm are formed on opposite sides of the recording layer, and a ZnS layer of 100 nm is formed on the SiN.sub.x layer on the side of the laser beam incidence and a ZnS layer of 200 nm is formed on the SiN.sub.x layer on the other side. There is described that the medium of such constitution exhibited good adaptability to repeated recording operations of several ten thousand times.
However, when the inventors of the present invention evaluated the medium of the layer constitution as described in the example of JP-A 64937/1990, the number of overwritable operations was only about 10,000.